Paja Formation Research Articles

A new species of Platypterygius (Ophthalmosauridae) from the lower Barremian of Colombia and assessment of the species composition of the genus

In this paper, we describe a new ophthalmosaurid specimen found in the lower Barremian beds of the Paja Formation at Villa de Leiva, Boyacá, Colombia. The specimen represents a new species of Platypterygius, P. elsuntuoso sp. nov. defined by three unique features within the genus: a small ventral peg in the anterior ventral border of the extracondylar area of the basioccipital; a supratemporal groove in the quadrate; and a shallow neck at the base of the acellular cementum ring on the teeth root. In support of our taxonomic determination, in this contribution we also assess the species composition of the genus Platypterygius, comparing the known anatomy of the type species P. platydactylus with that of the other species previously assigned to the genus. We found that a great morphological affinity in the forelimb of the species P. americanus, P. australis, P. hercynicus, and P. sachicarum (lower Barremian to lower Cenomanian) with that of the type species (Aptian) is unquestionable and differentiate them from all other ophthalmosaurids. Three new diagnostic features (synapomorphies of the genus) support their inclusion in Platypterygius. Consequently, the genus name “Kyhytysuka”, recently proposed to differentiate P. sachicarum from the genus Platypterygius is here rejected. A careful comparison of the cranial characteristics of Platypterygius specimens preserving associated cranial and fin remains demonstrate that Platypterygius is a genus distinguishable from other ophthalmosaurids not only by a typical forelimb but also by a particular combination of skull features. In this context, the skull morphology of “Simbirskiasaurus” concurs with that defining Platypterygius and its minor differences are not sufficient to support its generic distinction. The morphological comparisons and our phylogenetic analysis show the new species P. elsuntuoso more closely related to the Colombian species P. sachicarum than to other species of Platypterygius. Given that the new species came from the lower Barremian and P. sachicarum came from the upper Barremian, the minor differences in dentition found between the two species suggest a speciation by adaptation to new food sources in the Cretaceous epicontinental sea of Colombia.

Top of the food chains: an ecological network of the marine Paja Formation biota from the Early Cretaceous of Colombia reveals the highest trophic levels ever estimated

Abstract The Mesozoic Marine Revolution restructured the world’s ocean biodiversity into the complex marine ecosystems of today. This revolution began during the Triassic but the origin of this complexity is poorly understood due to a lack of detailed ecosystem reconstructions throughout time. We present the first site-specific ecological network for a marine Mesozoic fauna based on the Early Cretaceous Paja Formation biota of Colombia that preserves numerous, large-bodied, predatory marine reptiles. The trophic food-web was quantitatively reconstructed based on inferred trophic interactions of marine producers, consumers, and large apex predators. Compared to well-studied Caribbean reef ecosystem networks, the Paja biota network is missing a great proportion of benthic invertebrates and fishes, despite its rich higher trophic levels. We hypothesize that the ammonites from the Paja biota either mirrored the diversity represented by some fishes today or established a novel trophic unit with no living analogue. Recalibrating the Paja biota network to trophic analogues in the Caribbean, such as sea turtles, estimates that the largest Paja marine reptile hyper-apex predators occupied trophic levels a full tier higher than any extant marine apex predator. The Paja biota network is a starting point to tracing the evolution of marine ecosystems across the Mesozoic Marine Revolution.

Procumbent anterior premaxillary teeth in Stenorhynchosaurus munozi (Plesiosauria, Pliosauridae), evidence from new material

A recently prepared fragment of the anterior end of a snout comprising occluded upper and lower jaws of a juvenile individual of Stenorhynchosaurus munozi Páramo-Fonseca et al., 2016 (Plesiosauria, Pliosauridae) is described herein. The specimen was found in the 1990s at Sutamarchán (Boyacá, Colombia), in Barremian beds of the Arcillolitas abigarradas Member of the Paja Formation. Its description provides hitherto unknown valuable morphological information on the species: dorsal anterior extension of the vomer, five premaxillary teeth (not four as was previously suggested), and anteriorly directed orientation of the first premaxillary alveolus (procumbent). New observations made on previously studied material corroborated the existence of procumbent anterior premaxillary teeth in the species. The presence of this trait highlights the close relationship of S. munozi with other early-diverging brachauchenines from the Lower Cretaceous of Russia.

Integrated Characterization of Mudstones in the Andes of Colombia: Understanding Its Complexities for Risk Mitigation

This article presents a comprehensive vision of particularities and constraints of the Paja Formation in the Northern Andes of Colombia, supported by personal, institutional, and academic experiences, including a doctoral thesis in completion (geomorphology and risks research line). Such fine-grained marine rocks cause severe damage in diverse zones, with little spread, and are very unfavorable, especially within the Eastern Cordillera (departments of Santander and Cundinamarca), whose socio-environmental problems motivated a popular legal action in the municipality of Vélez due to the cracking and collapse of houses, damage to roads and landslides in the urban area, as well as flows, subsidence, and high hydrogeochemical dynamism or rare earths, although they also presented spontaneous ignition at the rural area. Understanding how these problems originate and interrelate is the main objective of the work. At the beginning, we include some brief definitions, terms, and key approaches to understand the consolidated geomaterials, location, and background of the problem; then, the results of meso–macro–micro studies, obtained by combining the field techniques and conventional instrumental laboratory analyses (tests on the chemistry of water and soil, description of samples with magnifying glasses, petrography with a polarized light microscope, micromorphology of regoliths–colluvions) of nanoscientists (emphasizing RXD-RXF, SEM, IR-Raman spectroscopy, TOC-TS) are presented. These characterizations and new knowledge must be socially and institutionally appropriated and applied in land use planning and risk management for the sustainability of challenging environments with the stratiforms of Lower Cretaceous rocks and associated Quaternary deposits in populated mountainous areas and contrasting intertropical hydroclimatological regimes, geologically active, so unstable and insecure.

First Report on the Geologic Occurrence of Natural Na–A Zeolite and Associated Minerals in Cretaceous Mudstones of the Paja Formation of Vélez (Santander), Colombia

This study reports for the first time the geologic occurrence of natural zeolite A and associated minerals in mudstones from the Cretaceous Paja Formation in the urban area of the municipality of Vélez (Santander), Colombia. These rocks are mainly composed of quartz, muscovite, pyrophyllite, kaolinite and chlorite group minerals, framboidal and cubic pyrite, as well as marcasite, with minor feldspar, sulphates, and phosphates. Total organic carbon (TOC), total sulfur (TS), and millimeter fragments of algae are high, whereas few centimeters and not biodiverse small ammonite fossils, and other allochemical components are subordinated. Na–A zeolite and associated mineral phases as sodalite occur just beside the interparticle micropores (honeycomb from framboidal, cube molds, and amorphous cavities). It is facilitated by petrophysical properties alterations, due to processes of high diagenesis, temperatures up to 80–100 °C, with weathering contributions, which increase the porosity and permeability, as well as the transmissivity (fluid flow), allowing the geochemistry remobilization and/or recrystallization of pre-existing silica, muscovite, kaolinite minerals group, salts, carbonates, oxides and peroxides. X-ray diffraction analyses reveal the mineral composition of the mudstones and scanning electron micrographs show the typical cubic morphology of Na–A zeolite of approximately 0.45 mμ in particle size. Our data show that the sequence of the transformation of phases is: Poorly crystalline aluminosilicate → sodalite → Na–A zeolite. A literature review shows that this is an unusual example of the occurrence of natural zeolites in sedimentary marine rocks recognized around the world.

A benchmark specimen of Muiscasaurus catheti from the upper Aptian of Villa de Leiva, Colombia: New anatomical features and phylogenetic implications

A new specimen of Muiscasaurus catheti Maxwell et al., 2016 from upper Aptian of the Paja Formation found in Villa de Leiva (Boyacá, Colombia) is described herein. It consists of a well preserved skull and a nearly complete axial skeleton of a sub-adult individual and provides valuable new morphological information on the species, including two autapomorphies: the presence of a quadratojugal with extremely reduced external exposure, and basioccipital floor of the foramen magnum with a medially furrowed anterior ridge. A detailed comparison between the new specimen (FCG-CBP-16) and the holotype (CIP-FCG-CBP-74) allows us to reinterpret some features of the original description of M. catheti that were obscured by the highly deformed and partially unprepared state of the holotype. Consequently, we propose the new specimen (FCG-CBP-16) to be a benchmark specimen for the species. Morphological comparisons with other Ophthalmosauridae highlight the affinities between M. catheti and the species of the Upper Jurassic genus Ophthalmosaurus. A phylogenetic analysis recovers M. catheti in a ‘basal’ position within Ophthalmosaurinae, supporting its proximity to Ophthalmosaurus and contrasting with previous topologies where M. catheti was recovered as a basal Platypterygiinae. This implies a temporal extension of the Ophthalmosaurinae up to the Aptian in South America.

Orithopsid crabs from the Lower Cretaceous Paja Formation in Boyacá (Colombia), and the earliest record of parasitic isopod traces in Raninoida

The Barremian–Aptian Paja Formation, as exposed in Villa de Leyva and nearby areas (Boyacá, Colombia), is one of the most fossiliferous Cretaceous units in northern South America, yet no fossil crabs had ever been recorded from this formation in the region. Here we report the first occurrence of crabs from the upper Aptian “Arcillolitas con nódulos huecos” Member of the Paja Formation in Boyacá, belonging to two species of orithopsid crabs: Bellcarcinus aptiensisLuque, 2014 and Planocarcinus olssoni (Rathbun, 1937). The newly collected material — represented by dozens of samples in small nodules — includes individuals preserving key rostral, orbital and pereonal traits previously unknown from the type material. Furthermore, a few specimens of B. aptiensis have swellings in their branchial regions, consistent with the deformation produced by parasitic isopods infesting the crab's gill chambers. To our knowledge, this is the first record of parasitic isopod traces in the family Orithopsidae, and the oldest evidence of isopod parasitism in raninoidan crabs. The new material here reported increases the number or marine arthropods known from the fossiliferous Paja Formation in Boyacá and provides valuable information for further palaeoecological reconstructions of these shallow-marine settings in northern South America during the Early Cretaceous.

Optical parameters and microstructural properties of Solid Bitumens of high reflectance (Impsonites). Reflections on their use as an indicator of organic maturity

Six solid bitumen samples from Paja Formation (Cretaceous of Colombia) were optically, chemically and structurally characterized using bulk chemistry, reflectance indicating surface (RIS), micro-Raman and X-ray diffraction (XRD) parameters. The volatile matter and carbon contents and high random reflectance (Ro%) placed these solid bitumens in the category of cata-impsonites according to Jacob's classification. RIS axes (RMAX, RINT and RMIN) form almost a sphere, and consequently, the anisotropy (Ram or bireflectance values) of these materials is very weak, comparatively to other carbonaceous materials (coals) of similar degree of evolution. The weak anisotropy character of the solid bitumen is related to the absence of pressure in the system, which did not promote the orientation of the basic structural units (BSUs) in a three-dimensional arrangement. The degree of ordering evaluated by Raman parameters, such as full width at half maximum (FWHM) of the D1 and G bands, and XRD parameters, including d002 and crystallite sizes Lc and La, showed that the solid bitumens of the Paja Formation have a low structural order equivalent to high-rank coals, specifically anthracites B (3.0% ≤ Ro < 4.0%), although their reflectance values placed them in a higher rank (Ro > 4.0%). The discrepancy between high reflectance and low degree of structural order can explain why some of the equations used to estimate equivalent vitrinite reflectance (Ro eq. Vite) from solid bitumens do not work universally. Taking in consideration several equations available in the literature, Jacob's equation (Jacob, 1989) was the one that fitted the Ro eq. Vite values in the range of 3.0% to 4.0%, which parallels with the degree of structural order obtained for the solid bitumens. Consequently, Jacob's equation can now be extended to solid bitumen in the range of cata-impsonites.

An Early Cretaceous Teleosauroid (Crocodylomorpha: Thalattosuchia) from Colombia

Teleosauroid extinction at the Jurassic/Cretaceous boundary has been hypothesized to be an artifact of sampling only temperate paleolatitudes. here, we describe the definitive youngest record of Teleosauroidea, from the upper Barremian of the paleotropics. The preserved material consists of dorsal vertebrae, dorsal ribs, dorsal and ventral osteoderms and epipodial remains. We refer the specimen to Teleosauroidea based on the large hourglass-shaped amphiplatyan morphology of the dorsal centra and rectangular dorsal osteoderms, which are much wider than long and have a straight anterior edge. The South American specimen is one of the largest known teleosauroids, with an estimated body length of 9.6 m. This is the first evidence of a marine crocodylomorph recorded from the Paja Formation of Colombia. The survival of Teleosauroidea in the paleotropics of northern Gondwana following the group’s extinction in europe supports the hypothesis that water temperature played a role in controlling the diversity and distribution of these large marine predators. doi: 10.5710/AMGH.26.09.2019.3269

Atherfieldastacus magnus (MʼCoy, 1849), a widely distributed crustacean during Early Cretaceous (Valanginian-Albian) times

Several specimens of the mecochirid lobster Atherfieldastacus magnus (MʼCoy, 1849) collected in Mexico and Colombia, represent the oldest record for this species. The Mexican specimens are from the Valanginian-Hauterivian San Juan Raya Formation, in Puebla, central Mexico, and the Colombian specimens were found in Barremian deposits of the Paja Formation, north of Bogotá. Previous reports of Atherfieldastacus magnus include numerous specimens from the Aptian of Europe (UK, Spain), Mexico, Colombia and Tibet. The new specimens collected in shales of the San Juan Raya Formation of Mexico, represent diverse sizes and excellent quality of preservation; in some samples the articles of the antennae can be observed, among other morphological characters not reported previously for this species. The association of the Mexican specimens with mollusks, echinoids and cones of an araucaria gymnosperm, suggest a shallow marine environment. The same is observed for the Colombian specimens, where sediments of the Paja Formation are reported as shallow marine. Additionally, sexual dimorphism in A. magnus is recognized, based on the analysis of differential morphology of the pleurae. Thus, for the moment, the biostratigraphic range for A. magnus ranges from the Valanginian to the Albian (Early Cretaceous), with the oldest occurrence in Mexico, where it persisted until Aptian times.

A new specimen of Stenorhynchosaurus munozi Páramo-Fonseca et al., 2016 (Plesiosauria, Pliosauridae), from the Barremian of Colombia: new morphological features and ontogenetic implications

ABSTRACTA new specimen of Stenorhynchosaurus munozi Páramo-Fonseca et al., 2016 (Plesiosauria, Pliosauridae), found at Sáchica (Boyacá, Colombia) in upper Barremian beds of the Arcillolitas Abigarradas Member of the Paja Formation, is described herein. It consists of a complete skull and 10 cervical vertebrae of an adult individual and provides valuable new morphological information on the species: the presence of nasals covering a subsurface circular cavity where salt glands were probably lodged (new autapomorphy), the presence of a high sagittal crest on the parietal, a very elongated pineal foramen enclosed entirely within the parietals, and unfused neural arches in the cervical vertebrae. A phylogenetic analysis supports the inclusion of Stenorhynchosaurus munozi within the pliosaurid clade Brachaucheninae. A morphological comparison between the juvenile and adult stages of S. munozi shows that in the adult stage, the margins of the internal nares and the anterior part of the anterior interpterygoid vacuity are well ossified, the medial joints of the pterygoids and the mandibular symphysis are closed, and the atlas and the axis are fused, but the cervical neural arches remain unfused to the centra. The growth pattern of the skull dorsal bones differs from that of the palatal elements, but the relative distance between the external and internal nares remains the same. A greater longitudinal growth in the skull and a negative allometric orbital growth pattern results in a proportionally longer skull and relatively smaller orbits in the adult.

A new late Aptian elasmosaurid from the Paja Formation, Villa de Leiva, Colombia

A new genus and species of elasmosaurid, Leivanectes bernardoi gen. et sp. nov., from the upper Aptian levels of the Paja Formation of Villa de Leiva (Boyacá, Colombia) is described. The new elasmosaurid is characterized by a short mandibular symphysis, bears only three alveoli (there are five in Callawayasaurus colombiensis), has an enlarged premaxillary alveoli, and has a mandible that includes only seven alveoli in each ramus anterior to the orbit (there are 11 in Callawayasaurus colombiensis). This new elasmosaurid taxon has fewer and larger alveoli than any other presently described elasmosaurid taxon. The observed differences indicate that the new species consumed larger-bodied prey than did other elasmosaurids. The new taxon suggests that the elasmosaurids were diverse in the Colombian late Aptian sea.

A new specimen of Platypterygius sachicarum (Reptilia, Ichthyosauria) from the Early Cretaceous of Colombia and its phylogenetic implications

ABSTRACTPlatypterygius sachicarum is one of the few Lower Cretaceous ichthyosaurian species described from the Hauterivian–Aptian-aged Paja Formation, the most complete Lower Cretaceous sedimentary sequence in northern South America. To date, P. sachicarum has been described only from a single skull, limiting morphological, stratigraphic, and phylogenetic comparisons. Here, we describe a new skull and associated postcranium of upper Barremian age from Villa de Leyva, Colombia, which represent the first documented postcranial remains of this species and enable detailed comparison with other Early Cretaceous ophthalmosaurid ichthyosaurs. Platypterygius sachicarum shares many morphological similarities with contemporaneous taxa from Europe but differs in both skull and tooth morphology from coeval South American species from northern South America and from the eastern Pacific. The additional data provide new diagnostic characters for this species and resolve the position of P. sachicarum as part of a polytomy of other species historically referred to Platypterygius. However, as with previous analyses, all recovered clades are poorly supported. The rich vertebrate fossil record of the Paja Formation provides an unparalleled opportunity to explore paleobiogeographic and paleoecological questions pertaining to Cretaceous marine reptiles; however, in most cases, a robust stratigraphic and phylogenetic framework remains elusive.

A new lamniform shark Protolamna ricaurtei sp. nov. from the Lower Cretaceous of Colombia

The Lower Cretaceous record of lamniforms from South America is scarce and poorly known, containing only two reports of isolated teeth from the upper Hauterivian of Argentina and the Albian of Peru. Here, we describe a partial articulated tooth set referable to †Protolamna ricaurtei sp. nov. from the upper Barremian–lower Aptian deposits of the Paja Formation (Andes of Colombia). The new species corresponds to one of the oldest fossil lamniforms to date reported from South America. This articulated tooth set is the only known for the family †Pseudoscapanorhynchidae, and possibly the oldest in its type for a lamniform from the Lower Cretaceous. Our findings offer new insights in to the lamniform paleodiversity of the northwestern margin of Gondwana during the Early Cretaceous.

The first South American sandownid turtle from the Lower Cretaceous of Colombia.

Sandownids are a group of Early Cretaceous-Paleocene turtles that for several decades have been only known by cranial and very fragmentary postcranial elements. Here I report and describe the most complete sandownid turtle known so far, including articulated skull, lower jaw and postcranial elements, from the Early Cretaceous (upper Barremian-lower Aptian, >120 Ma), Paja Formation, Villa de Leyva town, Colombia. The new Colombian sandownid is defined here as Leyvachelys cipadi new genus, new species and because of its almost identical skull morphology with a previously reported turtle from the Glen Rose Formation, Texas, USA, both are grouped in a single and officially (ICNZ rules) defined taxon. Phylogenetic analysis including L. cipadi supports once again the monophyly of Sandownidae, as belonging to the large and recently redefined Pan-Chelonioidea clade. The morphology of L. cipadi indicates that sandownids were not open marine turtles, but instead littoral to shallow marine durophagous dwellers. Leyvachelys cipadi not only constitutes the first record of sandowinds in South America, but also the earliest global record for the group.

A new ophthalmosaurid ichthyosaur from the Early Cretaceous of Colombia

AbstractCretaceous ichthyosaurs were relatively diverse in temperate latitudes, but few species have been described from the palaeotropics. Here, we describe a new ophthalmosaurid ichthyosaur, Muiscasaurus catheti gen. et sp. nov., from the Barremian–Aptian aged Paja Formation of Colombia. This species is known only from a partial skull and differs from all other ichthyosaurs in the unusual configuration of the external narial opening, slender rostrum, narrow postorbital region, and gracile dentition. It is the second ichthyosaur described from the Paja Formation, suggesting moderate taxonomic and ecological ichthyosaur diversity in the region during the Early Cretaceous.

A new Early Cretaceous brachiosaurid (Dinosauria, Neosauropoda) from northwestern Gondwana (Villa de Leiva, Colombia)

ABSTRACTBrachiosaurid sauropods achieved a broad distribution during the Late Jurassic, which has been considered to provide evidence of their origins during the Middle Jurassic, prior to the breakup of Pangea. In contrast to their broad geographic distribution during the Late Jurassic, formally named brachiosaurid species from the Cretaceous have so far been restricted to the Aptian–Albian of North America, which has been interpreted as a signal of differential extinction and/or a bias in the Early Cretaceous fossil record. Here we describe a new brachiosaurid titanosauriform taxon from the Early Cretaceous of Colombia, which is represented by axial elements. The material was recovered from marine sediments of the Paja Formation (Barremian), close to the locality of Villa de Leiva. The weakly laterally expanded and divided transverse processes of the anterior-most caudal vertebrae allows the recognition of a new sauropod taxon, Padillasaurus leivaensis, gen. et sp. nov. In order to test the phylogenetic relationships of the new taxon, we performed a cladistic analysis that recovered Padillasaurus as a brachiosaurid titanosauriform. This position is supported by a combination of characters, including the presence of blind fossae in anterior caudal vertebrae. Among titanosauriforms, the presence of blind fossae in anterior caudal vertebrae is an apomorphic character that is exclusive to Giraffatitan, Venenosaurus, Cedarosaurus, and Abydosaurus. Although more complete remains are needed to test more thoroughly the affinities of the new taxon, the available evidence indicates that brachiosaurids survived at lower latitudes in Gondwana until at least the Early Cretaceous.http://zoobank.org/urn:lsid:zoobank.org:pub:652D6B2A-7A8A-4311-8725-279BF2C9E0E3SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP

Geochemistry, palaeoenvironments and timing of Aptian organic-rich beds of the Paja Formation (Curití, Eastern Cordillera, Colombia)

Abstract At the Curití Quarry, Eastern Cordillera of Colombia, a 23.35 m section includes a 6.3 m interval of organic-rich marlstones, calcareous clay-shales and clay-shales devoid of benthic fossils and bioturbation, at the base of the Paja Formation. It overlies carbonate ramp deposits of the Rosablanca Formation, with the topmost layer containing reworked and phosphatized assemblages of middle Barremian to lowest Aptian ammonites of the genera Pulchellia , Gerhardtia , Toxancycloceras , Karsteniceras and Prodeshayesites . High-resolution analyses of the section, including total inorganic carbon (TIC, 0.09–50 wt%), total organic carbon (TOC, up to 8.4%) and stable carbon isotope measurements (δ 13 C org between −22.05‰ and −20.47‰) allow the determination of the stratigraphic relationship between the organic-rich level within the Paja Formation and Oceanic Anoxic Event-1a (OAE-1a). The range of values and the overall pattern of the δ 13 C org curve are comparable with the Lower Aptian interval C7. Thus, the organic-rich shale interval of the Paja Formation at the Curití section, showing characteristics of oxygen-depleted conditions, was deposited after OAE-1a, which is known to occur between isotopic levels C3 and C6. Supplementary material: TIC, TOC and Stable Carbon Isotope data at Curiti Section are available at http://www.geolsoc.org.uk/SUP18634

DEVELOPMENT OF EXPERIMENTAL CORRELATIONS BETWEEN INDENTATION PARAMETERS AND UNCONFINED COMPRESSIVE STRENGTH (UCS) VALUES IN SHALE SAMPLES

Unconfined Compressive Strength (UCS) is one of the rock mechanical properties that is important take into account during drilling operations in order to avoid wellbore instability. During drilling operations, UCS variability influences wellbore stability more than other factors such as azimuth, slope, exposure time, and mud weight (Jaramillo, 2004). In last years, the indentation technique has been demonstrated to be an appropriate method for determining rock strength in real time during oil well drilling. This technique implements correlation that allow UCS evaluation from indentation parameters such as Indentation Module (IM) and Critical Transition Force (CTF), that can be measured on small rock fragments obtained during drilling. Shale formations in well drilling have demonstrated to be a hindrance since they represent the most important problem in reservoir stability (Abass, H., A. et al. 2006). Therefore, the main objective of this article is to find experimental correlations that allow the modeling of rock strength by applying the indentation technique to reservoir plug. The importance of this technique is the possibility to get rock strength properties in real time during drilling operations, although, those reservoir sections which do not have neither direct UCS records nor indirect measurements. Eight Unconfined Compression Tests (UCS) on rock cylinders (plugs) extracted from the Paja formation upwelling were conducted in order to develop the corresponding experimental correlations. Two hundred indentation tests were also simultaneously conducted on shale fragments extracted from each plug surroundings in order to simulate the cavings obtained from reservoir drilled. Results of both tests were correlated using the Minimum Square technique, seeking the best correlation that shall represent result behavior, thus obtaining two 2nd-degree polynomial correlations. Correlation coefficients of 0,6513 were determined for the (IM) - (UCS) correlation and 0,8111 for the (CTF) - (UCS) correlation. This demonstrates that the highest correlation between indentation parameters and (UCS) is obtained with the Critical Transition Force (CTF).

Geochemistry of black shales from the Lower Cretaceous Paja Formation, Eastern Cordillera, Colombia: Source weathering, provenance, and tectonic setting

The major and trace element characteristics of black shales from the Lower Cretaceous Paja Formation of Colombia are broadly comparable with those of the average upper continental crust. Among the exceptions are marked enrichments in V, Cr, and Ni. These enrichments are associated with high organic carbon contents. CaO and Na 2O are strongly depleted, leading to high values for both the Chemical Index of Alteration (77–96) and the Plagioclase Index of Alteration (86–99), which indicates derivation from a stable, intensely weathered felsic source terrane. The REE abundances and patterns vary considerably but can be divided into three main groups according to their characteristics and stratigraphic position. Four samples from the lower part of the Paja Formation (Group 1) are characterized by LREE-enriched chondrite-normalized patterns (average La N/Yb N = 8.41) and significant negative Eu anomalies (average Eu/Eu ∗ = 0.63). A second group of five samples (Group 2), also from the lower part, have relatively flat REE patterns (average La N/Yb N = 1.84) and only slightly smaller Eu anomalies (average Eu/Eu ∗ = 0.69). Six samples from the middle and upper parts (Group 3) have highly fractionated patterns (average La N/Yb N = 15.35), resembling those of Group 1, and an identical average Eu/Eu ∗ of 0.63. The fractionated REE patterns and significant negative Eu anomalies in Groups 1 and 3 are consistent with derivation from an evolved felsic source. The flatter patterns of Group 2 shale and strongly concave MREE-depleted patterns in two additional shales likely were produced during diagenesis, rather than reflecting more mafic detrital inputs. An analysis of a single sandstone suggests diagenetic modification of the REE, because its REE pattern is identical to that of the upper continental crust except for the presence of a significant positive Eu anomaly (Eu/Eu ∗ = 1.15). Felsic provenance for all samples is suggested by the clustering on the Th/Sc–Zr/Sc and Gd N/Yb N–Eu/Eu ∗ diagrams. Averages of unmodified Groups 1 and 3 REE patterns compare well with cratonic sediments from the Roraima Formation in the Guyana Shield, suggesting derivation from a continental source of similar composition. In comparison with modern sediments, the geochemical parameters (K 2O/Na 2O, La N/Yb N, La N/Sm N, Eu/Eu ∗, La/Sc, La/Y, Ce/Sc) suggest the Paja Formation was deposited at a passive margin. The Paja shales thus represent highly mature sediments recycled from deeply weathered, older, sedimentary/metasedimentary rocks, possibly in the Guyana Shield, though Na-rich volcanic/granitic rocks may have contributed to some extent.